Organic light emitting diodes (OLEDs) typically include several layers of organic materials interposed between conductive thin film electrodes, with at least one of the organic layers being an electroluminescent layer. When a voltage is applied to electrodes, holes and electrons are injected from an anode and a cathode, respectively. The holes and electrons injected by the electrodes migrate through the organic layers to reach the electroluminescent layer. When a hole and an electron are in close proximity, they are attracted to each other due to a Coulomb force. The hole and electron may then combine to form a bound state referred to as an exciton. An exciton may decay through a radiative recombination process, in which a photon is released. Alternatively, an exciton may decay through a non-radiative recombination process, in which no photon is released. It is noted that, as used herein, internal quantum efficiency (IQE) will be understood to be a proportion of all electron-hole pairs generated in a device which decay through a radiative recombination process.
A radiative recombination process can occur as a fluorescence or phosphorescence process, depending on a spin state of an electron-hole pair (namely, an exciton). Specifically, the exciton formed by the electron-hole pair may be characterized as having a singlet or triplet spin state. Generally, radiative decay of a singlet exciton results in fluorescence, whereas radiative decay of a triplet exciton results in phosphorescence.
More recently, other light emission mechanisms for OLEDs have been proposed and investigated, including thermally activated delayed fluorescence (TADF). Briefly, TADF emission occurs through a conversion of triplet excitons into singlet excitons via a reverse inter system crossing process with the aid of thermal energy, followed by radiative decay of the singlet excitons.
An external quantum efficiency (EQE) of an OLED device may refer to a ratio of charge carriers provided to the OLED device relative to a number of photons emitted by the device. For example, an EQE of 100% indicates that one photon is emitted for each electron that is injected into the device. As will be appreciated, an EQE of a device is generally substantially lower than an IQE of the device. The difference between the EQE and the IQE can generally be attributed to a number of factors such as absorption and reflection of light caused by various components of the device.
An OLED device can typically be classified as being either a “bottom-emission” or “top-emission” device, depending on a relative direction in which light is emitted from the device. In a bottom-emission device, light generated as a result of a radiative recombination process is emitted in a direction towards a base substrate of the device, whereas, in a top-emission device, light is emitted in a direction away from the base substrate. Accordingly, an electrode that is proximal to the base substrate is generally made to be light transmissive (e.g., substantially transparent or semi-transparent) in a bottom-emission device, whereas, in a top-emission device, an electrode that is distal to the base substrate is generally made to be light transmissive in order to reduce attenuation of light. Depending on the specific device structure, either an anode or a cathode may act as a transmissive electrode in top-emission and bottom-emission devices.
An OLED device also may be a double-sided emission device, which is configured to emit light in both directions relative to a base substrate. For example, a double-sided emission device may include a transmissive anode and a transmissive cathode, such that light from each pixel is emitted in both directions. In another example, a double-sided emission display device may include a first set of pixels configured to emit light in one direction, and a second set of pixels configured to emit light in the other direction, such that a single electrode from each pixel is transmissive.
In addition to the above device configurations, a transparent or semi-transparent OLED device also can be implemented, in which the device includes a transparent portion which allows external light to be transmitted through the device. For example, in a transparent OLED display device, a transparent portion may be provided in a non-emissive region between each neighboring pixels. In another example, a transparent OLED lighting panel may be formed by providing a plurality of transparent regions between emissive regions of the panel. Transparent or semi-transparent OLED devices may be bottom-emission, top-emission, or double-sided emission devices.
While either a cathode or an anode can be selected as a transmissive electrode, a typical top-emission device includes a light transmissive cathode. Materials which are typically used to form the transmissive cathode include transparent conducting oxides (TCOs), such as indium tin oxide (ITO) and zinc oxide (ZnO), as well as thin films, such as those formed by depositing a thin layer of silver (Ag), aluminum (Al), or various metallic alloys such as magnesium silver (Mg:Ag) alloy and ytterbium silver (Yb:Ag) alloy with compositions ranging from about 1:9 to about 9:1 by volume. A multi-layered cathode including two or more layers of TCOs and/or thin metal films also can be used.
Particularly in the case of thin films, a relatively thin layer thickness of up to about a few tens of nanometers contributes to enhanced transparency and favorable optical properties (e.g., reduced microcavity effects) for use in OLEDs. However, a reduction in the thickness of a transmissive electrode is accompanied by an increase in its sheet resistance. An electrode with a high sheet resistance is generally undesirable for use in OLEDs, since it creates a large current-resistance (IR) drop when a device is in use, which is detrimental to the performance and efficiency of OLEDs. The IR drop can be compensated to some extent by increasing a power supply level; however, when the power supply level is increased for one pixel, voltages supplied to other components are also increased to maintain proper operation of the device, and thus is unfavorable.
In order to reduce power supply specifications for top-emission OLED devices, solutions have been proposed to form busbar structures or auxiliary electrodes on the devices. For example, such an auxiliary electrode may be formed by depositing a conductive coating in electrical communication with a transmissive electrode of an OLED device. Such an auxiliary electrode may allow current to be carried more effectively to various regions of the device by lowering a sheet resistance and an associated IR drop of the transmissive electrode.
Since an auxiliary electrode is typically provided on top of an OLED stack including an anode, one or more organic layers, and a cathode, patterning of the auxiliary electrode is traditionally achieved using a shadow mask with mask apertures through which a conductive coating is selectively deposited, for example by a physical vapor deposition (PVD) process. However, since masks are typically metal masks, they have a tendency to warp during a high-temperature deposition process, thereby distorting mask apertures and a resulting deposition pattern. Furthermore, a mask is typically degraded through successive depositions, as a conductive coating adheres to the mask and obfuscates features of the mask. Consequently, such a mask should either be cleaned using time-consuming and expensive processes or should be disposed once the mask is deemed to be ineffective at producing the desired pattern, thereby rendering such process highly costly and complex. Accordingly, a shadow mask process may not be commercially feasible for mass production of OLED devices. Moreover, an aspect ratio of features which can be produced using the shadow mask process is typically constrained due to shadowing effects and a mechanical (e.g., tensile) strength of the metal mask, since large metal masks are typically stretched during a shadow mask deposition process.
Another challenge of patterning a conductive coating onto a surface through a shadow mask is that certain, but not all, patterns can be achieved using a single mask. As each portion of the mask is physically supported, not all patterns are possible in a single processing stage. For example, where a pattern specifies an isolated feature, a single mask processing stage typically cannot be used to achieve the desired pattern. In addition, masks which are used to produce repeating structures (e.g., busbar structures or auxiliary electrodes) spread across an entire device surface include a large number of perforations or apertures formed on the masks. However, forming a large number of apertures on a mask can compromise the structural integrity of the mask, thus leading to significant warping or deformation of the mask during processing, which can distort a pattern of deposited structures.